In the operation of modern aircraft, flight control surfaces such as slats and flaps are powered by primary movers commonly known as power drive units. Typically, the power drive units generate drive torque which is transmitted via a variety of transmission means to move the flight control surfaces in desired directions depending on the navigational and other demands placed on the aircraft. Given the importance of the flight control surfaces to the safety of the aircraft and its passengers, it is critical that the flight control surfaces be controlled by a reliable actuation system.
One problem associated with many flight control systems is that if the drive line or transmission controlling the flight control surface becomes mechanically jammed, or if flight conditions prevent movement of the flight control surface in the desired direction, the power drive unit will not stop generating torque. Rather, the power drive unit, which is normally hydraulically powered, will generate relatively high stall torque. This stall torque will be transmitted to the input shaft and gearing of the flight control surface actuator, and is often sufficiently high to detrimentally affect and potentially seriously damage the flight control surface or the flight control surface actuator.
Examples of devices that successfully prevent the transmission of excessive drive torque by sensing an axial force on an output shaft including a ball screw that drives a flight control surface are disclosed in U.S. Pat. Nos. 4,318,304 to Lang; 4,459,867 to Jones; 4,697,672 to Linton; and 5,655,636 to Lang et al., the entire disclosures of which are incorporated herein by reference. While these devices have proven quite successful for the intended purpose, there is always room for improvement. For example, because these devices sense the total output force from the actuator, the preloaded springs that sense the output force must be of sufficient size to accommodate the total output force. This tends to impede weight and size reduction of such devices.
A device that successfully prevents transmission of excessive drive torques by using a ball ramp to sense torque, rather than an axial force on an output shaft, is disclosed in U.S. Pat. No. 5,299,666 to Lang et al., the entire disclosure of which is incorporated herein by reference. Again, while this device is satisfactory for its intended purpose, there is always room for improvements. For example, the use of a ball ramp tends to restrict options for arranging components within the actuator and, also tends to limit size reduction along the rotational axis of the ball ramp.